1. Field of the Invention
The present invention relates to filters, and particularly to a dual-band coupled-line balanced-to-unbalanced bandpass filter with stepped-impedance resonators (SIRs).
2. Description of the Related Art
With the rapid development of wireless communication technologies, radio-frequency (RF) transceivers operating at multiple separated frequency bands are needed. For example, high-speed wireless LANs need to operate at both 2.4 GHz and 5 GHz bands. To this end, devices such as dual band antennas, dual band baluns and dual band filters are gaining wide attention currently.
Among various functional passive devices, filters are considered as one of the most important components, and therefore most of the research efforts have been made upon this particular area, especially upon the configuration of filters that utilize stepped-impedance resonators (SIRs) to achieve dual band features.
Balun is a device for converting a balanced signal to an unbalanced one, or vice versa. A balanced signal consists of two signal components with the same magnitude but 180° out-of-phase. Many analog circuits, such as mixer, amplifier and multiplier, require a balanced input or output to achieve noise or high-order harmonics reduction.
A traditional coupled-line balun is formed of conductive tracks coupled to each other. The balun's operating frequency is conditioned by the line length. A Marchand balun is a symmetrical balun having its coupled lines calculated in λ/4, where λ represents the wavelength corresponding to the central frequency of the passband desired for the balun.
For single band applications, passive devices that combine both filter-type and balun-type functionalities have been proposed in order to miniaturize RF front-end system modules. However, no such device exists for dual band applications.
FIG. 1A shows the architecture of a traditional dual band RF front-end device, which includes a diplexer 800′ and two pairs of bandpass filters and baluns 901, 902 and 901′, 902′, respectively. One pair of bandpass filter and balun is used for one frequency band channel. This known architecture renders a complicated structure at the RF front-end and increases the cost of the related apparatus.